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If you need to compute an aggregated summary of statistics against data in different groups, you can use the aggregate and reshape functions to compute the summary statistics of data subsets:

1. Use aggregate to calculate the mean of each iris attribute group by the species:

        > aggregate(x=iris[,1:4],by=list(iris$Species),FUN=mean)  

1. Use reshape to calculate the mean of each iris attribute group by the species:

        >  library(reshape)
        >  iris.melt <- melt(iris,id='Species')
        >  cast(Species~variable,data=iris.melt,mean,
             subset=Species %in% c('setosa','versicolor'),
             margins='grand_row')

For information on reshape and aggregate, refer to the help documents by using ?reshape or ?aggregate.